Hyponatremia and Hypernatremia in Kidney Disease: What You Need to Know

Feb, 18 2026

When your kidneys start to fail, they don’t just stop filtering waste. They also lose their ability to keep your sodium levels in check. This can lead to two dangerous conditions: hyponatremia (too little sodium in the blood) and hypernatremia (too much). These aren’t just lab numbers-they’re real risks that can cause falls, confusion, seizures, and even death in people with chronic kidney disease (CKD). About 1 in 5 people with advanced CKD will develop one of these sodium disorders. And many of them don’t even know it until it’s too late.

How Your Kidneys Keep Sodium in Balance

Your kidneys are like smart water managers. They don’t just filter blood-they decide how much water and sodium to keep or flush out, based on what your body needs. In a healthy person, if you drink a lot of water, your kidneys make lots of dilute urine to get rid of the extra fluid. If you’re dehydrated, they hold onto water and make concentrated urine. This keeps your blood sodium level steady, usually between 135 and 145 mmol/L.

But in CKD, this system breaks down. As kidney function drops below 30 mL/min/1.73m² (Stage 4 or 5 CKD), the kidneys can no longer make enough dilute urine. That means they can’t get rid of excess water. At the same time, they struggle to conserve water when you’re low on fluids. The result? A narrow window for safe water intake. Drink too much, and sodium drops. Drink too little, and sodium rises.

This isn’t just about drinking water. It’s also about what you eat. Sodium comes from salt, but also from protein and other solutes in food. When doctors tell CKD patients to cut back on salt, potassium, and protein to protect their kidneys, they’re trying to reduce the workload. But here’s the catch: cutting solutes too much can make hyponatremia worse. Less solute means less ability to excrete water-even if you’re drinking the same amount.

Hyponatremia: The Silent Threat in CKD

Hyponatremia (sodium <135 mmol/L) is the most common sodium problem in CKD. It shows up in 60-65% of cases, mostly as euvolemic hyponatremia-meaning your body has normal fluid volume, but too much water relative to sodium. Why? Because your kidneys can’t make dilute urine anymore. Even if you drink 1.5 liters a day, your body can’t get rid of it. That water builds up, diluting the sodium in your blood.

Thiazide diuretics, often used for high blood pressure, make this worse. They stop working well when GFR drops below 30, but they still mess with sodium handling. Up to 30% of hyponatremia cases in CKD are linked to these drugs. And many patients don’t realize they’re at risk. A 2023 study of Japanese CKD patients found that those on strict low-sodium diets had higher rates of hyponatremia-not because they ate too much salt, but because they ate too little protein and other solutes, which reduced their kidneys’ ability to clear water.

The dangers are real. People with hyponatremia are 1.8 times more likely to die than those with normal sodium levels. They’re more likely to fall, break bones, or develop dementia. In hospitals, hyponatremia at admission raises death risk by 28%. If it develops while you’re in the hospital, the risk climbs even higher. It’s not just about the number-it’s about how fast it drops. Rapid correction can cause osmotic demyelination, a brain injury that leaves people paralyzed or locked-in. That’s why doctors limit correction to no more than 6 mmol/L in 24 hours for CKD patients.

Hypernatremia: When You’re Dehydrated and Don’t Know It

Hypernatremia (sodium >145 mmol/L) is less common but just as dangerous. It happens when you lose more water than sodium. In CKD, this often means not drinking enough. Elderly patients, especially those with dementia or mobility issues, may not feel thirsty or can’t get water on their own. Some medications, like diuretics or laxatives, make it worse. Others, like diabetes or high blood sugar, cause osmotic diuresis-pulling water out with urine.

The problem? Your kidneys can’t concentrate urine anymore. In healthy people, urine can get as concentrated as 1,200 mOsm/kg. In advanced CKD, it’s often stuck around 300 mOsm/kg-the same as blood. That means no matter how dehydrated you are, your kidneys can’t save water. You keep losing it.

Symptoms are subtle at first: dry mouth, fatigue, confusion. But as sodium climbs above 155 mmol/L, seizures and coma can follow. Correction must be slow-no more than 10 mmol/L in 24 hours-to avoid brain swelling. Giving too much water too fast can cause cerebral edema, especially in people who’ve been dehydrated for days.

Three Types of Hyponatremia in CKD

Not all hyponatremia is the same. In CKD, it breaks into three types:

• Hypovolemic hyponatremia (15-20% of cases): You’ve lost both salt and water, but lost more salt. This can happen with vomiting, diarrhea, or salt-wasting kidney diseases. Diuretics are a big culprit here.
• Euvolemic hyponatremia (60-65%): The most common. Fluid volume is normal, but water builds up because kidneys can’t excrete it. Think: drinking too much water on a low-solute diet.
• Hypervolemic hyponatremia (15-20%): You have too much fluid overall, usually from heart failure or severe edema. The extra fluid dilutes sodium.

Knowing which type you have changes everything. Giving IV fluids to someone with hypervolemic hyponatremia can drown them. Restricting fluids in someone with hypovolemic hyponatremia can crash their blood pressure.

What to Do: Treatment That Actually Works

There’s no one-size-fits-all fix. Treatment depends on your kidney stage, symptoms, and type of imbalance.

• For hyponatremia: Fluid restriction is first-line. But how much? In early CKD, 1,000-1,500 mL/day. In advanced CKD, drop to 800-1,000 mL/day. Cut back on low-solute foods like tea, soup, and fruit juice. Avoid thiazide diuretics if GFR is below 30. Sodium supplements (4-8 g/day) may help in salt-wasting cases. Never rush correction.
• For hypernatremia: Replace water slowly. Use oral fluids if possible. If IV fluids are needed, use 5% dextrose in water-not saline. Correct sodium by no more than 10 mmol/L per day. Check for causes: dehydration? meds? diabetes? Stop the trigger.

Medications like vaptans (which block water reabsorption) sound great-but they’re risky in CKD. Your kidneys can’t respond to them properly, and they can cause liver damage or worsen kidney function. Most guidelines now say to avoid them in Stage 4 and 5 CKD.

The Bigger Picture: Diet, Drugs, and Daily Life

Managing sodium in CKD isn’t just about numbers. It’s about daily habits. Most patients juggle four restrictions: sodium, potassium, protein, and fluid. That’s a lot. A 2020 study found it takes 3-6 visits with a renal dietitian just to understand what’s safe to eat. Many patients think “low-sodium” means “no salt,” so they avoid all salty foods-and then eat mostly fruits, tea, and rice, which are low in solutes. That’s how hyponatremia sneaks in.

Elderly patients are most at risk. They make up 70-75% of advanced CKD cases. Many don’t feel thirsty. Some can’t reach a glass of water. Others forget to drink. A simple solution? Keep water by the bed. Set phone alarms to drink. Use a marked cup to track intake.

Medication interactions are another silent killer. Diuretics, SSRIs, and painkillers can all mess with sodium. Always review meds with your pharmacist. And if you’re on dialysis? Your sodium levels bounce every session. That’s why continuous sodium monitoring patches-newly approved in 2023-are a game-changer. They track interstitial sodium in real time, helping doctors adjust fluid goals before you even feel sick.

What’s Next? New Tools and Better Guidelines

The 2024 KDIGO guidelines will likely shift how we manage sodium in CKD. Instead of fixed fluid limits, we’ll move toward personalized targets based on residual kidney function. A patient with GFR of 25 mL/min might need 900 mL/day. One with GFR of 45 might handle 1,400 mL. One-size-fits-all is outdated.

Research is also looking at the gut-kidney axis. Early studies suggest the intestines might help handle sodium when kidneys fail. Could probiotics or fiber help? We don’t know yet-but it’s a promising path.

The bottom line? Sodium disorders in CKD are preventable. But only if we stop treating them like simple lab values. They’re signs of a broken system. Fixing them means looking at diet, meds, habits, and kidney function together.

If you or someone you care for has CKD, ask your nephrologist: "What’s my current sodium level? Is my fluid intake too high or too low? Am I on a medication that could be making this worse?" Don’t wait for a crisis. Talk now.